Electrical prospecting method and apparatus



May 22, 1945. J. w. MILLINGTON 2,376,619

ELECTRICAL PROSPEGTING METHOD AND APPARATUS Filed Dec. 12, 1941' :ggsuunoun 66 a. 52

wig/522 Pate nted May 22, 194

ELECTRICAL rrtosrao'rmo METHOD AND APPARATUS John W. Millington, Beaumont, Tex., assignor to Sperry- Sun Well Surveying Company, Philadelphia, Pa., a corporation of Delaware Application December 12', 1941, Serial No. 422,669

' 5 Claims. to]. 175-182) This invention relates to geophysical prospecting, and more particularly, by so-called electrical coring, to the determination of the nature and boundaries of formations traversed by bore holes. In the patent to Bazzoni and Razek No. 2,167,630, dated August 1, 1939, there is described a method and apparatus for electrical prospecting, the method involving lowering into a bore hole an apparatus arranged to propagate high frequency oscillations into the strata surrounding the apparatus. By causing the oscillationproduclng apparatus to .be affected by the strata in its vicinity, a measurement of the electrical conditions of the apparatus will give an indication of the formations which are encountered. Instead of having the oscillation-producing apparatus affected by the field it produces, a separate detecting means may be provided to record,

for example, the intensity of the field in a given location in the vicinity, thereby also securing an indication of the strata through which the apparatus is passing. The various matters to be taken into account are fully described in said patent, and reference is made thereto for a more general description of the problems which arise.

In order to secure logs of most useful character, it is generally desirable to provide a plurality of logs indicative of variations of different properties of the strata. From the electrical standpoint, the outstanding properties of the strata which serve to differentiate and identify them are the resistivity, the dielectric constant. and the magnetic permeability. Two difierent kinds of rocks, for example, may have the same resistivity, but may be differentiated by reason of a diiference intheir permeabilities or dielectric constants. It has been proposed, accordingly, to log wells at low and high frequencies or at different high frequencies, inasmuch as the higher the frequency the more there enterinto the picture the effects of the dielectric constant and permeabilitiy as compared with the resistivity. One object of the present invention is to take into account the different effects which may result from the several electrical properties of the strata to secure a plurality oflogs for the purpose of better differentiating between strata than may be accomplished by the production of a single log. In the case of a preferred embodiment of the invention, this result is secured by causing an oscillator to be affected by different properties so as to change a plurality of characteristics of the oscillations produced thereby. This is done in such fashion that one characteristic is more responsive to a property, such as the dielectric constant, than the other characteristic, with the result that differences in the logs obtained are of significance.

A further object of the invention relates to theprovision of means for varying the degree of response of the apparatus to changes in the formav tions.

Further objects of the invention, and particularly those relating to details, will become apparent from the following description, read in conjunction with the accompanying drawing, in which:

Figure 1 is a wiring diagram illustrating a preferred embodiment of the invention;

Figure 2 is a record of the type produced by the use of the apparatus of Figure 1;

Figure 3 is a fragmentary diagram illustrating an alternative recording arrangement; and- Figure 4 is a diagram showing apparatus for the analysis of the record produced in accordance with Figure 3.

In view of the fact that the apparatus is for the general purposes illustrated in said Bazzoni and Razek patent, and since a substantially similar arrangement of parts within a bore hole may be used, there are illustrated herein only such details of the invention as are necessary to understand it.

Asillustrated in the Bazzoni and Razek patent, the apparatus, including a high frequency generator, the recording apparatus and all the associated parts, may be included within a casing adapted to be lowered through the bore hole without electrical connection to the surface. Alternatively, however, only the generating and detecting apparatus may be located within the casing or housing in the hole and signals transferred to recording apparatus at the surface. The power supplies for operation may be wholly within the hole, wholly at the surface, or in part within the hole and in part at the surface. The possibility of these various arrangements will be apparent hereafter. For-the sake of simplicity in description, however, it may be assumed that all of'the apparatus in Figure 1 is located together with its power supplies within a casing adapted to be lowered within the hole by means of a cable or wire line which is not called uponto conduct any current.

There is indicated at 2 a vacuum tube which may be of any suitable type, though preferably it is a pentode or a beam tetrode, in order to provide the possibility of control through a screen grid. Connected to the control grid of the tube, as indicated, is a tuned circuit comprising an inducnected to the plate there is another tuned cir'- cuit comprising coils 8, III and I2 in series connected in parallel with an adjustable condenser M. It will be obvious that the circuit thus arranged may constitute through the choice of proper values for the elements a tuned-plate tuned-grid oscillator which is designed to operate at high radio frequencies. The feedback necessary potentiometer l8 shunted across the plate supply voltage. In the present case, this type of control is very desirable, inasmuch as the major differentiation between the properties of the strata is obtained when the tube is just on the point of going out of oscillation. A substantial range of sensitivities is available by the use of the adjustment of the screen grid voltage. While the oscillator shown is of the tuned-plate tuned-grid type, it may equally well involve a conventional Hartley or Colpitts circuit, in which regeneration may be controlled in any suitable fashion, for

example through the medium of an adjustable screen grid voltage if a screen grid tube is used.

The coil l2 in the plate circuit is the exploratory coil of the logging apparatus and may be located below the casing or housing of the apparatus as described in said Bazzoni and Razek patent, or i it may take the form of a nosepiece, as illustrated in Bazzoni and Razek Patent 2,222,136, dated November 19, 1940. (See also Ellis and Millington Patent 2,197,493. dated April 16, 1940.) This coil l2 provides a high. frequency field producingv means and may be replaced by an antenna arrangement or by electrodes having electrical contact with the mud in the hole so as to provide the high frequency currents in the strata by direct conduction rather than by induction. The field producing means, furthermore, may be located in other parts of the circuit disclosed or in various parts of other oscillator circuits, as will be evident to those skilled in the art following the disclosures of the'various patents referred to above. The precise way in which the high frequency field is produced is immaterial from the standpoint of the principles of th invention. It is 'sumcient that the field-producing means, i, e., a coil, antenna or electrodes, should present to the oscillator a variable impedance so that both the frequency and amplitude of the oscillations produced by the oscillator are varied.

The coils 8 and I0, which may also be located in other portions of the circuit. serve to couple the circuit to responsive indicating or recording apparatus which may take the forms illustrated.

Coupled to the coil I0 is a secondary coil 20 which is, in turn. connected between the grid and cathode of a vacuum tube 22 biased to act as a detector. The plate of this tube is connected through a choke 24, high voltage supply and recorder 26 to its cathode. The recorder is merely diagrammatically illustrated at 26 and may take the form of a photographic or other recording galvanomtance 4 and an adjustable condenser 8. Contrace on the photographic strip. The dotted lines in the plate circuit of tube 22 indicate convenient places for the location of transmission lines if the recording is to be done at the surface rather than within the casing which is moved through the hole.

The net effect of th recording circuit just described is response to variations of amplitude of the. oscillations produced by the apparatus independently of variations in frequency, the input circuit of the detector being untuned.

To the coil 8 there is coupled a secondary coil 28 which provides a high frequency input to the automatic volume control arrangement indicated at 30, which may be of any conventional character serving to provide an output to the high frequency transformer 32 which is substantially constant in amplitude, but'varying in frequency, depending upon frequency variations of the oscillator. The output of the transformer 32 is fed to a discriminator circuit whichv may be of any type used for the detection of frequency modulated signals. As shown, it comprises a load arrangement bridging the secondary of the transformer, and consisting of a resistance 34 in series with a condenser 36. The voltage across the condenser 36 is fed to a diode 38 and through a low pass filter 40 to a recording galvanometer or amplifier-galvanometer arrangement, indicated diagrammatically at 42, which may be of any of the typ'es described above in connection with recorder 26. Here, again, the dotted lines indicate the possible location of transmission lines to the surface if surface recording is to be accomplished.

The circuit last described serves for indicating frequency variations in the oscillator output independently of amplitude variations. Thus, since the recording galvanometer 26 is independent of frequency variations by reason of the untuned input to the detector 22, one recorder will provide indications of frequency variations alone, while the other will produce indications of amplitude variations alone. These two recorders are arranged to produce records simultaneously on a single strip of film or paper, which is desirably fed either in synchronismwith the lowering of the apparatus within the hole or at a fixed rate of speed with the addition of time markings, as indicated, for example, in Bazzoni and Razek Patent 2,222,136. The recording operations may be essentially as described in my application Serial No. 344,227, filed July 6. 1940, or in the application of Bazzoni and Millington, Serial No. 344,192, filed July 6, 1940. The recording per se forms no part of the present invention, and is subject to any suitable variation, depending upon whether the recording is done in the hole or at the surface.

In Figure 2 there is illustrated the type of record produced in recording at the surface, this record comprising a strip 44, carrying curves 46 and 48, produced respectively by the recording devices 26 and 42. Depth marks 50 may be provided in conventional fashion under the control of apparatus connected to the cable, so that correlation of the records with the depths at which they are produced may be accomplished. If the recording is done within the hole, an amplitude record may be produced carrying timing marks to make possible correlation of the curves with depth by reference to records of depth versus time provided at the surface.

In the operation of this apparatus, variations of impedance will be presented to the oscillator by reason of the changes of the material in the vicinity of the coil I2 or equivalent antenna or electrodes. These variations of impedance will involve both resistive and reactivecomponents.

Each of these will produce variations of both amplitude and frequency in the oscillator, particularly marked if it is operating on the verge of instability. However, contributions to frequency and amplitude variations are different for the resistive and reactive'components of the impedance presented, and consequently, while the records produced by the frequency and amplitude responsive meters will follow each other in .a general way, as indicated in Figure 2, the differences between the records are of significance in differentiating the variations in relationship of the resistive and reactive components of the effective impedance. While this difference in the traces may be read off from the curves such as 46 and 48 produced by deflection, galvanometers, differences may also be determined and made to stand out more sharply by recording and analysis of the records in accordance with Figures 3 and 4.

In Figure 3 and 4 there are illustrated the recording and analyzing of a record in fashions generally similar to those disclosed in said Bazzoni and Millington application Serial No, 344,1?2, referred to above.

Referring first to the recording operation, the apparatus therefor is diagrammatically illustrated in Figure 3. The two recorders 26 and 42 of Figure 1 would now be replaced by an amplifying arrangement, if necessary, delivering its output to lamps 52 and 54, beams from the filaments of which would be concentrated through a suitable optical system diagrammatically indicated as lenses 58 and 60 upon a travelling photographic film B4 to produce, upon developmenavariable density traces, indicated at 66 and 68. Simultaneously timing markings may be produced on the film through the medium of a lamp 5B controlled by clockwork mechanism and projecting a beam through lens 62 to produce a trace ill.

The variable traces 66 and 68 are, of course, not

readily analyzed by visual observation, and recourse is had to photoelectric scanning, which is effective not only to analyze the amplitude variations of the traces, but their difference as well.

The analyzing apparatus is illustrated in Figure 4 and comprises a series Of lamps 12 which, through optical systems concentrating their beams upon the traces 66, 68 and 10, are designed to illuminate in varying fashions photoelectric cells 16, 18 and 80. These cells are diagrammatically illustrated as of the self-generating type, though this, it will be understood, is merely diagrammatic, since'any photocell arrangement with or without amplification may be used. As illustrated specifically, the cells 16 and 18 are connected in series with each other and with a galvanometer 82 and adjustable balancing resistance 84. shunted between a point intermediate the cells and a point between the meter 82 and resistance 84 is another recording galvanometer 86. These galvanometers may be provided with moving mirrors in the usual fashion, designed to refiect beams oflight from lamps 88 upon a photographic paper or film 89 to trace curves 90 and 92. Simultaneously, the photocell 80 picks up the 'beam from the timing trace l0 and'through the medium of lamp 9B and galvanometer 94 records a timing trace indicated at 98.

It will be evident from the circuit described that the galvanometer 82 will respond substantially to the sum of the voltages generated at the photocells 16 and 18. Since these two traces will be rather similar, the galvanometer may be regarded as responding, in effect, to one of them.

' The galvanometer 86, on the other hand, which may involve a substantially greater recording amplication than the galvanometer 82, will serve to recordthe differences between the responses of the two photocells l6 and I8 tracing a curve 92 of these differences; By suitable setting of the resistance 84, balancing may be effected to such extent as-to eliminate from the responses of the galvanometer 86 substantially all of the corresponding responses of the photocells leaving the difference trace 92, which generally bears very little resemblance to the trace 90. Thus two formations which may give approximately the same type of deflection of the curve 90 may give wholly different deflections of the curve 92 so as to be readily distinguished from each other.

It will be obvious that various modifications of the arrangements disclosed fall within the scope of the invention.

What I claim and desire to protect by Letters Patent is:

1. Means for determining the location and character of formations penetrated by a bore hole comprising means for establishing an electromagnetic field penetrating formations in the vicinity of the bore hole, said field establishing means being arranged to be run within and lengthwise of the bore hole, a generator of high frequency oscillations supplying said field establishing means, said field establishing means and generator-being interconnected so that both the fashions characteristic of properties of said materials, and means for separately indicating visibly and quantitatively and in the same run said frequency and amplitude variations of the oscillations.

2. Means for determining the location and character of formations penetrated by a bore hole comprising means for establishing an electromagnetic field penetrating formations in the vicinity of the bore hole, said field establishing means being arranged to be run within and lengthwise of the bore hole, a generator of high frequency oscillations supplying said field establishing means, said field establishing means and generator being interconnected so that both the frequency and amplitude of oscillations produced by the generator are varied by earth materials in the vicinity of the field establishing means in fashions characteristic of properties of said materials, and means for separately indicating visibly and quantitatively and in the same run said frequency and amplitude variations of the oscil lations, said means for indicating said frequency variations comprising a circuit coupled to said generator and having an automatic amplitude control to provide an output of high frequency oscillations at substantially constant amplitude and means variably responsive to frequency variations of said output.

3. Means for determining the location and character of formations penetrated by a bore hole comprising means for establishing an electromagnetic field penetrating formations in the vicinity of the bore hole, said field establishing means being arranged to be run within and lengthwise of the bore hole, a generator of high frequency oscillations supplying said field establishing means, said field establishing means and generator serving to provide oscillations varying in both frequency and amplitude due to changes in the impedance of said field establishing means resulting from earth materials in the vicinity thereof, and means for separately indicating visibly and quantitatively and in the same run said frequency and amplitude variations of the oscillations.

4. Means for determining the location and character of formations penetrated by a bore hole comprising means for establishing in formations in the vicinity of the bore hole a single current wave having oscillations varying in both frequency and amplitude due to changes in said formations along the hole, and means for separately 

